Existing commercial transport jet aircraft typically include two or more primary turbine engines for propulsion. These aircraft also typically include at least one auxiliary power unit (APU) that provides power in addition to or in lieu of the power provided by the primary engines. Accordingly, APUs can be used to provide power to the aircraft when the primary engines are not running, for example, while the aircraft is waiting at an airport gate. The APUs can also provide temporary power to start the primary engines during normal operations, and/or temporary emergency power during an engine-out condition or other emergency condition.
Over the course of time, aircraft manufacturers and airlines have come under increasing pressure to reduce the noise emitted by aircraft during normal operations, including gate operations, taxi, take-off, and landing. Accordingly, aircraft manufacturers have developed mufflers and other sound-attenuating devices to reduce the noise emitted by both the aircraft primary engines and the APUs. One challenge associated with APU mufflers is developing internal structures that are both lightweight and capable of withstanding the large temperature gradients between the hot APU exhaust flow within the muffler, and the cold external environment associated with typical aircraft cruise altitudes. Accordingly, there is a need for APU exhaust duct structural arrangements that are both lightweight (to improve overall aircraft efficiency) and capable of withstanding large thermal gradients without buckling or otherwise becoming damaged.